Splined couplings are used in a wide variety of applications to transmit torque and rotary motion between input and output drive members. A common form of splined coupling comprises external splines formed on a radially outer surface of a first shaft and internal splines formed on a radially inner surface of a second shaft. The splines interengage so as to transmit torque but also allow for some relative axial movement between the shafts and for some limited angular deflection between the shafts.
One application for such a splined coupling is in an actuator drive system in an aircraft wing. An aircraft wing typically has multiple actuators used to deploy control surfaces, for example flaps or slats. Power is transmitted to the actuators from a central power drive unit via a transmission line which inter alia includes a number of splined couplings arranged between adjacent drive shafts. The splined coupling allows for some relative axial movement between the shafts, which accommodates changes in the length of the transmission line as might arise upon flexing of the aircraft wing. A problem which may arise in practice is that during assembly of the coupling, it is not always possible to accurately locate the coupling components. Thus if, for example, the splines are fully engaged upon assembly, they may not be able to slide relative to one another and thus accommodate the relevant movement.